A biofilm is a complex aggregation of microorganisms marked by the secretion of a protective and adhesive matrix. Biofilms are also often characterized by surface attachment, structural heterogeneity, genetic diversity, complex community interactions, and an extracellular matrix of polymeric substances.
Single-celled organisms generally exhibit two distinct modes of behavior. The first is the familiar free floating, or planktonic, form in which single cells float or swim independently in some liquid medium. The second is an attached state in which cells are closely packed and firmly attached to each other and usually a solid surface. The change in behaviour is triggered by many factors, including quorum sensing, as well as other mechanisms that vary between species. When a cell switches modes, it undergoes a phenotypic shift in behavior in which large suites of genes are up- and down-regulated.
Biofilms are usually found on solid substrates submerged in or exposed to some aqueous solution, although they can form as floating mats on liquid surfaces. Given sufficient resources for growth, a biofilm will quickly grow to be macroscopic. Biofilms can contain many different types of microorganisms, e.g. bacteria, archaea, protozoa and algae; each group performing specialized metabolic functions. However, some organisms will form monospecies films under certain conditions.
The biofilm is held together and protected by a matrix of excreted polymeric compounds called EPS. EPS is an abbreviation for either extracellular polymeric substance or exopolysaccharide. For the purpose of this application, EPS will mean exopolysaccharide. This matrix protects the cells within it and facilitates communication among them through biochemical signals. Some biofilms have been found to contain water channels that help distribute nutrients and signalling molecules. This matrix is strong enough that under certain conditions, biofilms can become fossilized.
Bacteria living in a biofilm usually have significantly different properties from free-floating bacteria of the same species, as the dense and protected environment of the film allows them to cooperate and interact in various ways. One benefit of this environment is increased resistance to detergents and antibiotics, as the dense extracellular matrix and the outer layer of cells protect the interior of the community. In some cases antibiotic resistance can be increased 1000 fold (Stewart and Costerton 2001).
Biofilms are ubiquitous. Nearly every species of microorganism, not only bacteria and archaea, have mechanisms by which they can adhere to surfaces and to each other.
Biofilms can be found on rocks and pebbles at the bottom of most streams or rivers and often form on the surface of stagnant pools of water. Biofilms are important components of food chains in rivers and streams and are grazed by the aquatic invertebrates upon which many fish feed.
Biofilms grow in hot, acidic pools in Yellowstone National Park (USA) and on glaciers in Antarctica.
In industrial environments, biofilms can develop on the interiors of pipes, which can lead to clogging and corrosion. Biofilms on floors and counters can make sanitation difficult in food preparation areas.
Biofilms can also be harnessed for constructive purposes. For example, many sewage treatment plants include a treatment stage in which waste water passes over biofilms grown on filters, which extract and digest organic compounds. In such biofilms, bacteria are mainly responsible for removal of organic matter (BOD); whilst protozoa and rotifers are mainly responsible for removal of suspended solids (SS), including pathogens and other microorganisms. Slow sand filters rely on biofilm development in the same way to filter surface water from lake, spring or river sources for drinking purposes.
One widely recognized health problem associated with biofilms is that they are present on the teeth of most animals, where they may become responsible for tooth decay.
In addition to tooth decay, biofilms have also been found to be involved in a wide variety of microbial infections in the body, by one estimate 80% of all infections (NIH 2002). Infectious processes in which biofilms have been implicated include common problems such as urinary tract infections, catheter infections, middle-ear infections, gingivitis, coating contact lenses, and less common but more lethal processes such as endocarditis, infections in cystic fibrosis, and infections of permanent indwelling devices such as joint prostheses and heart valves. (Lewis 2001, Parsek and Singh 2003).
Such bacterial infections are a persistent problem in human health. Outside of the body there are several means used to control reservoirs of infection including chemical disinfectants and forms of high-energy electromagnetic radiation e.g. ultraviolet light and X-rays. Although effective at controlling environmental populations, they cannot be used to treat bacterial pathogens once infection has occurred. To date, the only treatment that is known to be effective is antibiotics. The way antibiotics generally works is to take advantage of the variant metabolic pathways that exist between humans and bacteria, thereby, differentially affecting bacterial cells. They have two big drawbacks. First, they are not specific against any one type of bacteria and can damage commensal or beneficial bacteria resulting in new pathologies. Second, bacteria have readily evolved to become resistant to antibiotics. Since antibiotics are not beneficial to the bacteria, they can be neutralized without a loss of any critical functions. In addition, antibiotics are not very effective against a bacterial infection that has formed a biofilm.
Therefore, there still exists a need for an improved method to treat biofilm-related bacterial infections as well as to manage the formation of biofilms.